In one aspect the present invention relates to a sensitizing agent for explosive compositions which comprises coarse grade particulate aluminum coated with a finely divided hydrophobic fumed silica material. In another aspect, the subject invention relates to a process for producing a sensitizing complex useful in aqueous gel explosive compositions which comprises a relatively large air bubble attached to a particle of coarse grade particulate aluminum. In still a further aspect, the invention of the subject application relates to novel aqueous gel explosive compositions sensitized with coarse grade particulate aluminum coated with a finely divided hydrophobic fumed silica material.
Particulate aluminum has been used as an additive in conventional explosive compositions to a large extent in either coarse particulate form or in a finer particulate form known within the industry as "paint grade" aluminum. Compared to the large surface area per unit weight of paint grade aluminum (generally in the range of from 3 to about 10 square meters per gram) coarse grade aluminum, which typically includes atomized aluminum, for example, has a relatively small surface area per unit weight. For example, atomized aluminum normally has surface areas ranging from about 0.05 to about 1 meters square per gram.
Coarse grade particulate aluminum, such as atomized aluminum for example, has conventionally been used in explosive compositions in amounts in the range of from about 5 to 20% by weight of the explosive composition for the purpose of increasing the explosive energy of the compositions by providing a readily oxidizable fuel. On the other hand, relatively fine particulate aluminum, such as paint grade aluminum coated with stearic acid is recognized to impart a significant sensitizing effect in water gel explosive compositions. For example it is known that aqueous gel explosive compositions which are difficult to detonate, or are nondetonatable, can be made cap sensitive by adding paint grade aluminum in amounts ranging from about 2 to 5% by weight of the explosive composition. One accepted theory explaining the differences in effect on explosive compositions which occur when paint grade aluminum is employed as opposed to atomized aluminum, for example, is that the greater surface area of paint grade aluminum, when coated with stearic acid, allows a thin layer of air to be absorbed or carried thereon thus providing an air-aluminum combination which is very effective as a sensitizing agent. Coarser grades of aluminum such as atomized aluminum do not have surface areas as large, on the average, as those of paint grade aluminum. For example paint grade aluminum, which is always coated with a thin layer of stearic acid, may have surface areas in the range of from 3 to about 10 sq. m/g. In contrast, the surface area of atomized aluminum is usually in the range of only about 0.1 to about 0.3 sq. m/g.
Thus particulate aluminum in two distinct forms has been used in explosive compositions to obtain two different desirable properties. Very fine particulate aluminum, such as paint grade aluminum, has been employed as a sensitizing agent either alone or in conjunction with other chemical sensitizers such as perchlorates and amine nitrates. Coarser grades of particulate aluminum, such as atomized aluminum, have been added in order to increase the power of explosives by providing a high energy fuel source.
As noted above, paint grade aluminum is coated with stearic acid in order to provide a hydrophobic surface which will hold or absorb air, in a thick gelled medium, so as to provide the aluminum-air complex necessary for sensitization. However, the sensitizing effect of coarse aluminum is very small even if the surface contains a coating of hydrophobic stearic acid. Apparently this is because the amount of air absorbed is dependent upon the surface area of the aluminum and therefore the absorbed air is relatively small because of the relatively small surface area of coarse grade aluminum.
While paint grade aluminum does provide an effective sensitizing agent in many cases it is relatively expensive as compared to other sensitizing agents and, because of its very fine particle size, presents handling difficulties when employed in large scale explosive manufacturing processes. Therefore, a relatively inexpensive, easier to handle aluminum sensitizing agent is desirable.